A known diagnostic technique used in tomography for locating tumors involves injecting into a patient's bloodstream a radioactive isotope which targets the tumor, so that the location of the tumor can be derived by detecting the location of the radioactive isotope. Typically, the radioactive isotope emits high energy .gamma.-rays which are dispersed from the tumor site. In order to achieve the desired detection so as to determine the precise location of the tumor, it is necessary to image the patient's body in such a manner as to detect only those .gamma.-rays which are emitted normally from the body and to ignore those .gamma.-rays which are dispersed in other directions.
Known prior art approaches to achieving this requirement, include the use of a mechanical collimator made out of lead having a plurality of spaced apart holes which are sufficiently narrow in diameter to allow only those .gamma.-rays to pass which are emitted parallel to the collimator holes. The collimator is moved until a signal is detected whereupon the location of the collimator allows the location of the radioisotope to be inferred. However, since most of the radioactive energy is dispersed and therefore not detected, such an approach is highly inefficient and the detector requires lengthy exposure time which is expensive in terms of the time required to perform a reliable measurement as well as being uncomfortable for the patient. The resolution of such a system depends on the diameter of the holes in the collimator and is typically 8 mm. It can be improved be reducing the diameter of the holes in the collimator at the expense of decreasing even further the efficiency which is in any case typically no better than 10.sup.-5.
It is obviously desirable to reduce the measurement time as far as possible without compromising on the detection accuracy. This requirement has been partially addressed by the use of a Compton camera using ring geometry so that scattered photons are detected by the ring rather than being lost as is the case with mechanical collimators. This obviates the need for a collimator and allows the angle of emanation of the .gamma.-rays to be calculated. For the purpose of the present invention which is not concerned with the physics of the Compton effect, the Compton camera may be regarded as just another type of 2-dimensional image sensor having a plurality of addressable pixels, one of which emits a signal when stimulated by a .gamma.-ray. Specifically, each pixel is a diode which generates a charge signal when hit by a .gamma.-ray. A .gamma.-ray emitted by the radioisotope will be detected only if it creates a Compton effect by creating a charge signal thereby giving up some of its energy. In practice, it is usual to employ a composite sensor having several spaced-apart sensor layers each containing at least one sensor module so as to increase the probability that an incident .gamma.-ray will produce a Compton effect in at least one of the layers. The multilayer sensor module constitutes a first detector of the Compton camera. Having thus produced a Compton effect, the .gamma.-ray then emerges from the first detector. However, in order to calculate the angle of the incident .gamma.-ray, the emergent .gamma.-ray is directed to a second detector in which it is absorbed completely, thereby giving up all of its residual energy.
As a result of such a geometry, it is necessary to read out data in the first detector from a large number of addressable pixels along respective channels in order to detect which pixel is "active". This is done by first integrating the charge associated with each pixel using an integrator in the form of an operational amplifier (OP AMP) having a feedback capacitor. The integrated charge pulse is then amplified and shaped and the resulting analog signal is sampled and held, allowing its magnitude to be measured. In order to measure the peak magnitude of the shaped signal, the shaped signal must be very accurately sampled at the peak value. This requires an accurate determination of the peak time which occurs a fixed time difference t.sub.P after the emission of charge by the excited pixel. The fixed time difference t.sub.P is a function of the RC time constant of the shaper circuit and is therefore known.
Thus, in order to know when to sample the integrated charge signal, the time of occurrence t.sub.o of each charge emission must itself be accurately determined. This having been done, all that is then necessary is to sample the held integrated charge sample at a time t.sub.P. A reading system for reading out the charge signals must therefore generate an accurate trigger coincident with the occurrence of each charge emission. Self-triggering systems are known in which the channel in which the charge emission occurs generates the trigger by means of a level-sensitive discriminator. The pulse height is also latched so that it can be read out. However, such a system provides information regarding the pulse height only in the specific channel in which the charge emission occurred and not in other channels, except sometimes in the nearest neighboring channels. Moreover, no data is provided relating to the time of occurrence of the charge emission.
It is also known to generate the trigger by means of a separate electronic device on the common "back plane" of the image sensor. However, such an arrangement constrains the image sensor to being a "single-sided" detector rendering it impossible to determine where, in the sensor, the charge emission occurred, as well as being impractical to implement.
Obviously, if during every scan of the composite image sensor, each pixel is read sequentially only one at a time, then the current scan can be terminated when an "active" pixel is detected. However, it is impractical to read each pixel in such a manner because of the time overhead involved in addressing each pixel separately and downloading the pixel data along a dedicated channel for further processing. Furthermore, it will be appreciated that in addition to the one pixel with which the .gamma.-ray stimulation is associated, the other pixels too emit noise. Such noise may occur, for example, owing to the common mode drift of the OP AMPs associated with the reading circuit. When pixels are read only one at a time, it is difficult to quantify accurately the common mode noise component in the "active" pixel. Such considerations militate against addressing each pixel separately and favor batch addressing of a plurality of pixels in a single read operation using multiple channels each in respect of a corresponding pixel. This adds to the expense of the reading circuit, since various components thereof must be repeated for each channel. Having thus read a large number of data signals on separate channels each in respect of one pixel in the image sensor, it is then necessary to process the data in order to determine which pixel is "active", whereupon the location of the radioisotope may be inferred.
Moreover, as explained above, a non-zero common mode noise signal is associated with all of the pixels, including the "active" pixel. In order to measure the "active" pixel data accurately, the average common mode noise must be determined and subtracted from the "active" pixel data itself. This adds to the processing time and, obviously, the more pixels are processed simultaneously, the more time-consuming is the required processing.
Consequently, there is trade-off between reading the data sequentially pixel by pixel, with the consequent high addressing time and inability to compensate for common mode noise; and reading too many pixels simultaneously, with the consequent high processing time and added expense.
Yet a further consideration relates to establishing time coincidence of .gamma.-ray stimulated emissions in the two parallel detectors of a Compton camera. As has been explained above, in order to calculate the angle of the incident .gamma.-ray, the emergent .gamma.-ray from the first detector is directed to a second detector in which it is absorbed completely, thereby giving up all of its residual energy. It is obviously necessary to correlate events in the two detectors in order to establish that they derive from the same .gamma.-ray. This is done by establishing that the two events are substantially simultaneous. However, accurate time coincidence of the two events can be determined accurately only if the .gamma.-ray emission is measured fast. Prior art detectors employ a filter having a slow time constant for shaping the data signal resulting from the .gamma.-ray emission. A slow time constant is necessary to improve signal to noise ratio and to improve locking on to the peak value of the shaped signal. However, using a slow time constant detracts from the accuracy with which the peak time can be measured and this, in turn, reduces the accuracy with which time coincidence of corresponding events in two detectors can be established.
There is therefore clearly a need to optimize the reading of an array of pixels in a 2-dimensional image sensor so as to reduce the time taken to detect a single "active" pixel. Associated with this need is the requirement to provide an accurate trigger when a charge emission occurs so as to determine the time of emission (and thus the peak time) accurately thereby permitting time coincidence of events in more than one detector to be properly established, and to eliminate the effect of common mode noise from "active" pixel data so that only the actual data is read.